Molded zigzag resistor and method of making



Oct. 6, 1959 c. H. KRELLNER MOLDED ZIGZAG RESISTOR AND METHOD OF' MAKING 2 Sheets-Sheet 1 Filed Sept. 50, 1955 FIGS FIG.v 7

MA nulllll ATTR/VEYS Oct. 6, 1959 c. H. KRELLNl-:R 2,907,971

MOLDED zIGzAG REsIsToR AND METHOD oF MAKING Filed Sept. 30, 1955 2 Sheets-Sheet 2 28 JZ -W/ff/ N Mun l "M L HIIIIII' "'HH 'IMU LI' Ll \J' \.J'

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.dll 6 74 arf y MH mm FIG. 41 7 27 u 1 l \i Y Il 2# Z4 L wm 'wai INVENTOR. k z D Mmmm BYCLEMENT H.KRELLNER AT1-0R NE YS nited States Patent O MOLDED ZIGZAG RESISTOR AND METHOD F MAKING Clement H. Krellner, Bradford, Pa., assignor to Speer Carbon Company, St. Marys, Pa., a corporation of New York Application September 30, 1955, Serial No. 537,626 l 7 Claims. (Cl. 1338-293) This invention relates to electric resistors and t'o a method of manufacturing such resistors. More particularly the invention relates to molded, high resistance, high voltage, zigzag resistors which are encased in an integrally molded insulating cover, and to the method of manufacturing molded zigzag resistors.

Recent developments in television and related electronic fields have created a demand for high resistance, high voltage resistors. For example, resistors having values of the order of 50 to `100 megohms, and even higher, with 10,000 volts applied, are useful in color and other television circuits as voltage dividers for the electron guns of the cathode ray tubes. Such resistors should have a low voltage-resistance coefficient, i.e. they should have a minimum change of resistance Iwith change in voltage. Serious problems have been encountered in providing a long resistance path of small cross section which has good heat stability `and a low voltage-resistance coefficient. The resistors also should be relatively unaffected by atmospheric conditions, small in size, suitable for use in conjunction with printed circuits, capable of mass production, and economical to manufacture.

The long resistance path is a characteristic of zigzag resistors, but the zigzag resistors which have been available prior to the present invention have not met the other specifications recited above. In the past, zigzag resistors commonly have been made from a rod, slab, or plate of resistor material in which a plurality of staggered slots have been cut from opposite edges to produce a long zigzag resistance path. Where the resistor material is metallic, resistors have been cast in zigzag shape. Sometimes zigzag resistors, after having been formed in one of the manners described, have been encased within protective insulating coverings. The procedures of the prior art are not adapted to the economical manufacture of resistors of the type with which the present invention is concerned.

It is yan object of the present invention to provide an improved high resistance, high Voltage resistor. It is another object of the invention to provide a molded zigzag resistor encased within an integrally molded insulating covering. Itis another object of the invention to provide a high resistance, high voltage resistor which has a low voltage-resistance coeiiicient, which is small in size, which is adapted for use with printed circuits, which has good heat stability, and which is relatively unaffected by atmospheric conditions. It is still another object of the invention to provide a molded zigzag resistor which has a long resistance path of small cross section and uniform density throughout. It is yet another object of the invention to provide a resistor of the nature described which is capable of mass production and which may be manufactured more economically than resistors currently available for such usage. Other objects, improvements and advantages of the invention will become apparent or will be pointed out as the description proceeds.

A preferred embodiment of the invention selected for purposes of illustration and description is shown in the accompanying drawings, wherein:

Figure 1 is a perspective of a zigzag resistor made in accordance with the present invention;

lFigure 2 is an end elevation of the resistor illustrated in Figure l;

Figure 3 is a longitudinal section through this resistor taken substantially on the line 3 3 of Figure 2;

Figure 4 is a perspective of a preformed molded insulating shell used in forming resistors of the type illustrated in Figures l3;

Figure 5 is a longitudinal section through the insulating shell of Figure 4, corresponding to the section through the completed resistor shown in Figure 3;

Figure 6 is an exploded view of the parts of illustrative molding apparatus for preforrning the insulating shell shown in Figures 4 and 5;

lFigure 7 is a longitudinal section, to enlarged scale as compared ywith Figure 6, showing the parts of Figure 6 assembled and with the mold filled with insulating material preparatory to molding the insulating shell shown in Figures 4 and 5;

Figure 8 is an exploded view of illustrative molding apparatus for completing the molding of the zigzag resistor by molding the resistor material within the shell; and

Figure 9 is a fragmentary longitudinal section, to enlarged scale as compared with Figure 8, showing the parts of Figure 8 assembled and with the preformed insulating shell charged with resistor material preparatory to the iinal molding operation.

The material used for forming the insulating body within which the zigzag resistor is encased by molding is a iinely divided mixture of iiller and binder, for example, of silica our and a thermosetting resin. rl`he material used for the zigzag resistor path is a iinely divided mixture of filler, conductor and binder, for example, of silica flour, calcined carbon black and a thermosetting resin. The proportion of carbon black included in the resistor mixture will depend on the value of the resistance to be obtained, the length and cross sectional area of the resistor path, and possibly other factors. The resin employed may, for example, be a phenol-formaldehyde thermosetting resin. These insulating and resistor mixtures may have compositions similar to those heretofore used in the manufacture of molded resistors.

As can be seen in Figures 1 3 of the drawings, in external appearance the zigzag resistor of the present invention is in the form of a relatively thin rectangular parallelepiped designated generally at y11. Merely' for convenience in drawing, the resistor has been shown relatively thicker than it normally would be made. Exten nally, the resistor presents a continuous surface of electric insulating material Within which the zigzag resistor path is entirely embedded and encased. The resistor terminal leads 12 extend from the resistor path, in which their ends are embedded, through the insulating covering to permit the making of electric connections to the resistor.

In the drawings, 14 represents a preformed insulating shell having a series of parallel, evenly spaced openings d5 extending therethrough from one edge to the opposite edge. The walls separating adjacent openings are shortened alternately at the said two edges to form crossovers 116 between openings and provide a zigzag path wholly within the outlines of the shell. These openings 15 and the Crossovers l5 are charged with a moldable conducting mixture 17 of high electrical resistance and are closed at both edges of the shell with -a moldable insulating material indicated at 18, all molded together under pressure and cured to polymerize or set the resin in the insulating and conducting mixtures.

' through the spaced openings '24.

rFurther understanding of the construction of the resistor will be obtained from the following description of the illustrative apparatus for making the resistor, and of the description of themethod of manufacturing the resistor, reference being had particularly to Figures 6-9 of the drawings.

The apparatus for molding the preformed insulating shell is disclosed in Figures 6 and 7. In the drawings, 19 is a mold having a rectangular cavity 2li extending therethrough. This mold and the other parts of the molding apparatus to be described may be made of steel.

The bottom plate 21 has a plurality of pairs of parallel core rods 22 projecting upwardly therefrom. These-core rods are aligned and equally spaced from each other, the

space between rods ordinarily being less than the diameter of the rods. ln the illustrative embodiment there are of these core rods. i

The bottom punch 23 comprises a base plate 25 and secured on the top surface thereof an inverted T member 26. The vertical portion of the T member 26 lits the cavity 2li of the mold19 with a close sliding lit. The bottom punch '23 has a row of spaced openings 24 therethrough to receive the core rods 22 with close sliding lits. Between certain of these openings are frusto-pyramidal projections 27 that will form the Crossovers 16 between alternate openings 1S, there being four of these projections in the illustration located, respectively, between the second and third, the fourth and fifth, the sixth and seventh, and the eighth and ninth, openings. This leaves the first and tenth openings without crossovers, to receive the terminal leads 12.

The top punch 2S is generally similar to the bottom punch 23 except that, in the illustration, it has ve of the frusto-pyramidal projections 29, and these projections 29 connect the first and second, the third and fourth, the fifth and sixth, the seventh and eighth, and the ninth and tenth, openings. As in the case of the bottom punch, these frusto-pyramidal projections 29 serve to form `Crossovers 16 between the openings 15.

- The procedure for molding a preformed insulating shell with the apparatus disclosed in Figures 6 and 7 will now be described. T he bottom punch 23 is placed on the bottom plate "21, the core rods '22 passing upwardly The bottom punch is pushed all of the way down until it engages .the bottom plate 21. The mold 19 then is placed over the upper part of the bottom punch and pushed down until it rests Von parallel spacers 31 inserted between the hase plate of the bottom punch and the lower edges of the mold 19. The height of these spacers 31 determines the amount of insulating mixture which can be filled vinto the mold "19 on top of the bottom punch 23, surrounding the core rods 22, to ll the cavity 26 to the top of the mold.

Moldable insulating material then is poured into the cavity of the mold 19, around the core rods 22. The mold is tapped lightly or otherwise vibrated to settle this mixture and to eliminate air pockets. Excess insulating material then is removed until approximately level with the top of the mold 19.

The top punch 255 thenris threaded onto the core rods 22 and pushed downwardly until the insulating mixture is confined in the mold cavity 2u between the top and bottom punches. The whoie assembly then is placed on the platen of a press and light pressure is applied to force the top punch into the mold cavity for a short distance. The spacers 31 then are removed from between the base plate of the bottom punch "23 and the mold E19, allowingT the mold to operate free and give the effect of a floating mold.

A second pair of spacers islplaced between the base plate of the bottom punch and the top plate 33 of the top punch 28, straddling the sides of the mold 19. Pressure then is applied against the bottom plate 21 and the top plate 33 to compress the inSulating material between ,theV

i l punches 23 and 28 until further compression is halted by the spacers straddling the mold. These spacers determine the final height of the preformed insulating shell, which in turn determines the amount of resistor mixture which will be required for charging the insulating shell. The pressure will be such that the molded Vinsulating shell will retain its form during the succeedingoperations about to 'be described.

The bottom plate 21 then yis withdrawn from the assembled structure, the top punch 28 is withdrawn from the mold 19, andthe bottom punch 23 vis withdrawn from the rnold. This leaves the preformed insulating -shell 14 within the cavity 20 of the mold 19. Before continuing with the rdescription of the method of forming the molded zigzag resistor, -illustrative apparatus for that operation will be described, reference being had particularly to Figures 8 and 9 of thedrawings.

Figure 8 shows the mold 19 with the preformed insulating shell therein. A bottom punch 34 comprises a base plate 35 and an inverted T member 36 secured thereon.y The vertical portion of the T member fits the cavity 20 of the mold 19 with a close sliding 'fit and is provided with openings to receive the wires 12 or other terminals which are to be embedded in the resistor path. in the illustrative embodiment the punch 34 is provided with two such openings, located opposite the first and tenth, or end, openings 15 through the `preformed insulating shell. The shape of the terminal leads 12 may vary considerably, the drawings showing by way of illustration wire leads having their ends reversely bent. The top punch 37 is similar to the bottom punch 34 except that it is not provided with openings to receive terminal leads.

. Continuing the description of the molding operation, terminal leads 12 are inserted in the openings in the punch 34 and the punch then is pressed into the bottom of the cavity 20 in the mold 19, which contains the preformed molded insulating shell. The exposed ends of the terminal leads 12 desirably have been coated with a mixture of graphite and resin to' insure good electrical contact with the resistance material. The exposed ends of the terminal leads projecting above the top surface of the punch 34 kenter the first and tenth, ie. the twoend openings 15 through the preformed insulating shell. Pressure then is applied to the punch 34 and the mold 19 until the top of the preformed insulating shell 1'4 isllush with the top of the mold 19. Spacers may be inserted vbetween the base plate 35 and the mold 19 at this stage.

Into the tops of the openings 15 extending through the preformed insulating shell, and the Crossovers 116, is poured a small quantity of moldable insulating material which is sufficient to provide an insulating seal around the terminal leads 12 and insulating closures for the bottom Crossovers between the openings 15. The mold may be tapped lightly or otherwise vibrated to insure uniform distribution of the insulating material against the surface of the punch 34. While the thickness of this sealing layer may vary considerably, aV thickness of the order of 1/1f,-" Vis given as an illustrative example for resistors herein described.

Moldable resistor mixture then is poured into the top of 4the mold to fill the openings extending from edge to edge through the insulating shell and the crossovers kto within a short distance of the top edge of the resistor shell. A suitable vdistance may be ofthe order of lAG". The mold againiis vibrated to insure uniform filling of the openings 1S and crossovers 1'6,Y and removal of air pockets. j

Next, more of the insulating mixture is filled into the tops of the top Crossovers "116 to cover the resistormixture. The mold Vmay ybe vibrated and excess material scraped olf to leave the `insulationflush `withthe 'top-of the mold. 1g

The top punch37 :then .is v.placed against the ltop of the preformed insulating shell, now charged with resistor mixture and closing layers of insulating mixture, and isv pressed slightly into the mold cavity, for example by use of a hydraulic press. If spacers were inserted between the bottom punch 3-4 and the mold 19 they are now removed to leave the mold floating. Pressure on the punches 34 and y37 then is increased to the desired molding pressure, for example of the order of 180 pounds per square inch, depending on the materials used and other factors.

The punch '34 then is pressed out by exerting pressure on the punch 37 and the mold 19. This pressing is continued until the molded resistor is forced out of the cavity 20 of the mold 19. At this stage the molded resistor is green and uncured.

The green resistor then is cured or baked in a suitable oven in much the same manner that other types of molded resistors are cured. The temperature and time of curing are interrelated and both depend on the type of resin, the mixtures employed, and the dimensions of the resistor. After curing, the resistors are tested, classified, impregnated with waxes or resins in known manner to make them moisture resistant, and then are ready for use.

It will be understood that the hand molding apparatus shown in the drawings is merely illustrative. In the illustrative embodiment the resistor has two terminals embedded in opposite ends of the zigzag resistor path. If desired, additional connections intermediate the ends of the resistor path could be made by placing additional suitably located terminal receiving holes in the bottom punch 34.

Merely by way of illustration, and not limitation, zigzag resistors have been made on apparatus generally similar to that illustrated in Figures 6-9 and having dimensions which will now be set forth. The steel mold 19 had a cavity 20 about 35/32" long by wide extending through the mold, which was 3%" high. The bottom plate 21 had 10 core rods 22 which were 7/32 in diameter and about long with about 3x52" space between diameters. The '.tinal molding pressure was about 180 pounds per square inch. The iinished resistor had a height of 1% and a measured resistance of 105 megohms.

It will be understood that the invention herein disclosed may be variously modified and embodied within the scope of the subjoined claims.

@I claim as my invention:

1. A molded, high resistance, high voltage, zigzag resistor comprising, in combination, a preformed, integral, relatively thin rectangular parallelepiped insulating shell having parallel sides joined by a plurality of evenly spaced parallel walls forming a series of parallel evenly spaced openings extending through the shell from one edge to the opposite edge, the two end walls extending from edge to opposite edge, the other walls extending to one edge and terminating a predetermined distance from the opposite edge, every other one of 'these last mentioned walls extending to one of the said edges, and the intervening walls extending to the opposite edge, thereby to form crossovers between openings at the said edges and provide a zigzag path within the outlines of the shell, the said openings and the crossovers therebetween being charged with a conducting mixture of high electrical resistance compacted and bonded to form a continuous, zigzag resistance path within the insulating shell, and terminals of high electrical conductivity embedded in the opposite ends of the zigzag resistance path.

2. A resistor according to claim 1, including integral molded closures of insulating material in the ends of the openings and the crossovers therebetween over the high resistance conducting material.

3. A resistor according to claim 2, in which the said integrally molded insulating material substantially iills the openings and the crossovers to the boundary surfaces of the shell, and in which terminals of high electrical conductivity are embedded in the opposite ends of the zigzag resistance path and project through the overlying insulating material for the making of electrical connections to the resistor.

4. In the manufacture of molded, high resistance, high voltage, zigzag resistors the method which comprises molding from thermosetting insulating material a selfsustaining, relatively thin, rectangular parallelepiped insulating shell having a series of parallel evenly spaced openings extending therethrough from one edge to the opposite edge, with the walls separating adjacent openings being shortened alternately at the said edges to form crossovers between openings and provide a zigzag path within the outlines of the shell, charging the openings and the crossovers with a thermosetting conducting mixture of high electrical resistance, compressing the shell and the conducting material contained therein along the longitudinal axes of the openings into self-sustaining form, and curing the self-sustaining form to cause the thermosetting insulating and conducting materials to set.

5. .The method according to claim 4, in which the openings and the crossovers are not charged with conducting material flush with the said edges of the insulating shell, said method including the step of completing the filling of the openings and the crossovers with thermosetting insulating material substantially flush with the said edges.

6. In the manufacture of molded, high resistance, high voltage, zigzag resistors the method which comprises molding from thermosetting insulating material a selfsustaining, relatively thin, rectangular parallelepiped insulating shell having a series of parallel evenly spaced openings extending therethrough from one edge to the opposite edge, with the walls separating adjacent openings being shortened alternately at the said edges to form crossovers between openings and provide a zigzag path within the outlines of the shell, positioning the shell with the said openings substantially vertical, closing the bottoms of the openings, spreading a layer of thermosetting insulating material in the bottoms of the openings and crossovers, nearly filling the openings and crossovers with a thermosetting conducting mixture of high electrical resistance, covering the conducting material with a layer of thermosetting insulating material, compressing the shell and the materials assembled therein along the longitudinal axes of the openings into self-sustaining form, and curing the self-sustaining form to cause the thermosetting insulating and conducting materials to set.

7. 'Ihe method according to claim 6, including the step of positioning terminal wires with their ends projecting into the two openings which constitute the ends of the zigzag path before spreading the layer of insulating material in the bottoms of the openings.

References Cited in the le of this patent UNTTED STATES PATENTS 1,298,421 AThomson Mar. 25, 1919 1,767,715 Stoekle June 24, 1930 1,821,822 Wiegand Sept. l, 1931 1,987,969 Parkin Ian. l5, 1935 2,302,564' Megow et al. Nov. 17, 1942 2,454,508 Herrick et al. Nov. 23- 1948 

